The aim of this proposal is to determine the structure of AdiC, a member of a widespread and little-understood class of membrane transport proteins - the APC superfamily. APC proteins, nearly ubiquitous in the biological world, are used to move amino acids, polyamines, and a variey of organic cations across cell membranes for a multitude of physiological purposes. Although this is a huge molecular superfamily, no high-resolution structures are known for any APC proteins, and our understanding of their fundamental mechanisms of substrate transport is currently shallow. The bacterial APC protein AdiC, which moves arginine into many enteric bacteria, is a key player in the extreme acid resistance response that these organisms use to survive exposure to the acid environment of the stomach. This particular APC protein may be overexpressed, purified, functionally reconstituted in artificial membranes, and, most importantly, crystallized. The project aims to determine the structure of AdiC by x-ray crystallography, and to use this structural information to understand how this protein specifically recognizes its organocationic substrates and how it moves threse across the biological membrane.